Consolidate C++ proto files under src/proto

- make the directory structure match the package structure
- sanitize the package structure (test --> testing)
- add auto-detection of proto file dependencies
diff --git a/test/cpp/qps/server_sync.cc b/test/cpp/qps/server_sync.cc
index 3a15bec..97a1ff5 100644
--- a/test/cpp/qps/server_sync.cc
+++ b/test/cpp/qps/server_sync.cc
@@ -45,7 +45,7 @@
 
 #include "test/cpp/qps/server.h"
 #include "test/cpp/qps/timer.h"
-#include "test/proto/benchmarks/services.grpc.pb.h"
+#include "src/proto/grpc/testing/services.grpc.pb.h"
 
 namespace grpc {
 namespace testing {